Media were quickly shuffled out of the room during a meeting between Malaysia and New Zealand when Malaysian Prime Minister Mahathir Mohamad started airing concerns over China's actions in the South China Sea.

The meeting took place in Singapore at the East Asia Summit. Normally journalists are present for reasonably bland pleasantries between leaders during formal meetings, but the 93-year-old Malaysian leader was extraordinarily frank about his concerns.

"Although New Zealand is quite far from the South China Sea, what happens in the South China Sea will eventually impact all countries," he said.

Mr Mohamad continued saying the issue was very important to him and he hoped that all other countries would show interest.

"We hope that by everyone showing interest in this need for freedom of navigation and trade China might realise it is wrong of them to do anything that may upset navigation.

"While we accept that warships can come there, they should not be stationed there."

Prime Minister Jacinda Ardern and her deputy Winston Peters were visibly uncomfortable that he was discussing matters of significance in front of journalists.

Mr Peters soon began whispering to Ms Ardern's chief of staff Mike Munro, and the Prime Minister then signalled to New Zealand officials that they should get the media out of the room.

But when pressed about why she was uncomfortable, the Prime Minister brushed it off.

"It was merely just a discussion about his position on the South China Sea," she told New Zealand media after the meeting.

"Look, usually we have a bit of introductory comments and then the media leave - it was just obvious to me that the Prime Minister decided that he was going to just keep on going and get stuck into the substantive part."

"We had to make a decision at some point that the media might tire of standing there taking images - it was just a natural opportunity."

New Zealand was the designated host of the meeting and therefore it was Ms Ardern's responsibility manage how long the media stayed in the room.

"I just turned around and gave a little nod that now was probably the appropriate time for us to get on with the meeting."

Ms Ardern says she didn't take the Malaysia Prime Minister's comments as a warning to New Zealand, saying he had shared the same concerns during plenary meetings with all leaders present.

"What the Prime Minister of Malaysia said in our meeting was exactly the same as what he said in plenary it wasn't specific to us at all," said Ms Ardern.

China has recently stepped up its rhetoric around the South China Sea. Last month, Chinese President Xi Jinping told his military to "prepare for war".

Beijing has claimed territorial rights to the South China Sea, and there are suspicions that the strengthening of this military section is likely a signal to the US and any parties that the waters are being guarded by Beijing.

The area, through which about US$5 trillion in ship-borne trade passes each year, is contested by Brunei, Malaysia, the Philippines, Taiwan and Vietnam.